A known frictional propulsion device for an omni-directional vehicle comprises a main wheel including an annular core member and a plurality of driven rollers (free rollers) fitted on the annular core such that each driven roller may be freely rotatable around the tangential direction of the annular core member at the corresponding position, and a pair of drive disks provided on either side of the main wheel each including a disk member and a plurality of drive rollers (free rollers) arranged along the outer periphery of the disk member and engaging the driven rollers of the main wheel so as to be each rotatable around a rotational center line at an angle to both the lateral direction and the radial direction. See W008/132779A, JP2013-237327, JP2011-63209A and JP2013-107575A, for instance.
In the disclosed frictional propulsion devices, the drive disks are rotatably supported by the body frame of the omni-directional vehicle and are individually actuated by separate electric motors so that the vehicle can be propelled in the fore and aft, oblique and lateral directions. In the prior art disclosed in W008/132779A and JP2013-237327, the electric motors are positioned above the main wheel, and the power of the electric motors are transmitted to the respective drive disks by belt transmission mechanism. In the prior art disclosed in JP2011-63209A and JP2013-107575A, the electric motors are disposed coaxially to the drive disks so that the power of the electric motors are directly transmitted to the respective drive disks via respective shafts.
When the electric motors are positioned above the main wheel, the gravitational center of the vehicle is located at a relatively high position, and the overall height of the vehicle is inevitably great. When the electric motors are provided coaxially with respect to the drive disks, the gravitational center may be lowered, and the overall height of the vehicle may be reduced, but a gear reduction system of a high gear ratio and a compact design is required in view of minimizing the lateral dimension of the motor/gear reduction system assembly. Such gear reduction systems may be available but are costly and/or may lack the desired durability.